1. Field of the Invention
The present invention relates to a light-diffusing sheet to be used as a component of the screen of projection television or as a component of the liquid crystal display unit. The light-diffusing sheet permits one to watch clearly the image of a liquid crystal panel projected onto the front or rear side of the screen. Also, the light-diffusing sheet permits one to watch the image of a liquid crystal panel at a wider viewing angle.
2. Description of the Prior Art
The CRT projection television of conventional type is being replaced by another projection television of new type which is designed such that the image of a liquid crystal panel is projected onto a screen (from its front or rear side) and enlarged thereon by means of a projection lens. The rear-projection screen for such projection television is usually a lenticular lens sheet which is constructed of a clear plastic sheet and lenticular lenses formed thereon, as disclosed in Japanese Patent Laid-open No. 61120/1993 and others. The front-projection screen for such projection television usually has its surface coated with glass or plastic beads.
In the field of information machine such as personal computer and word processor, the liquid crystal display unit has come into general use recently. The liquid crystal used for the liquid crystal display unit is either of twisted nematic type (TN) or super twisted nematic type (STN). They poses a problem with narrow viewing angle. One way to address this problem is by installing a lens array on the viewer's side of the liquid crystal panel, thereby enlarging the image emanating from the liquid crystal panel, as disclosed in Japanese Patent Laid-open Nos. 25399/1978 and 65175/1981. Another way is by installing a light scattering plate in place of the lens array, as disclosed in Japanese Patent Laid-open Nos. 143791/1986 and 148430/1986.
A problem existing in the rear-projection screen provided with a lenticular lens sheet is with the occurrence of a moire pattern, which depends on the intervals of lenticular lenses and the size of pixels in the liquid crystal panel. The smaller the pixels are, the more prone the moire pattern is to occur. On the other hand, the beads-coated front-projection screen suffers the disadvantage that beads diffuse the projected light in random directions. So it is difficult to accomplish the desired viewing angles in both the horizontal and vertical direction.
The liquid crystal display unit consisting of a liquid crystal panel and a lens array placed thereon (toward the viewer) to enlarge the viewing angle poses a problem with the occurrence of a moire pattern if the lens array has a large lens diameter. Since a moire pattern occurs when there exists a particular relation between the intervals of the lens array and the dimensions of pixels of the liquid crystal panel, it is necessary that the lens diameter of the lens array should be smaller than the dimensions of pixels if the moire pattern is to be avoided. However, such a lens array has to be small in size, and it would be very difficult to make a mold for it. A light scattering plate as a substitute for the lens array is easy to produce, but it has the disadvantage of scattering light outside the required viewing angle, thereby lowering the image contrast.
The present invention was completed in order to address the above-mentioned problems. Accordingly, it is an object of the present invention to provide a light-diffusing sheet which offers advantages in different application areas as follows. When used for the rear-projection television, it prevents the occurrence of a moire pattern. When used for the front-projection television, it makes it possible to establish the viewing angle in the horizontal and vertical planes independently. When used for the liquid crystal display unit, it makes it possible to enlarge the viewing angle while preventing the image contrast from decreasing.